1. FIELD OF THE INVENTION
This invention relates to a transfer apparatus and method for transferring a web from a press section to a first dryer of the dryer section. More particularly, this invention relates to a transfer apparatus which includes a vacuum generating means disposed adjacent to the dryer nip for inhibiting wrinkling and breakage of the web.
2. INFORMATION DISCLOSURE STATEMENT
The transfer of a wet web from the press section to the dryer section of a papermaking machine has become a critical area in the production of light-weight grades of paper at high speeds. The web is usually transferred between the press section and the dryer section in an "open draw". In such "open draw", the sheet remains unsupported for a finite length between the press and dryer sections, and such unsupported lengths are subject to web flutter, breakage, and wrinkling as the web enters the dryer felt nip area.
In the prior art, several methods and arrangements have been proposed in an effort to minimize the aforementioned wrinkling. One method includes increasing the speed differential between the press section and the dryer section. Such difference between the press section and dryer section speeds tends to tighten the web, but it also tends to increase the susceptibility of the web to tension breaks.
In other proposals, the lead-in felt roll is moved in very close to the press roll in an effort to minimize the length of the "open draw". Such minimization of the "open draw" reduces the exposure of the web to external air currents; nevertheless, the web still tends to separate from the felt and cause wrinkling of the web as the web attempts to enter the nip between the dryer felt and the dryer surface.
Such separation of the web from the supporting felt has been the subject of several patent applications directed towards means for urging the web into contact with the felt.
U.S. Pat. No. 3,526,574 to Amend teaches a vacuum transfer box. Also, U.S. Pat. No. 4,551,203 to Eskelinen describes a blowing type vacuum box. Additionally, U.S. Pat. No. 4,698,919 to Wedel discloses an eductor type transfer box.
The problem with each of the aforementioned boxes is that the vacuum induced by these boxes must be high enough to evacuate all of the air between the web and the felt before the felt nip. Such high vacuum tends to pull the felt into contact with the boxes and this physical contact between the felt and the boxes tends to wear out the felt and the box seals, and can cause the web to locally separate from the felt.
The problem with the prior art devices stems from a failure to recognize the primary cause of the separation of the web from the felt. More particularly, the prior art devices have assumed that the separation is caused primarily by centrifugal forces and by gravity. However, the real cause is a "nip rejection" phenomenon. More specifically, the slight amount of air which naturally remains in-between the felt and the web is expelled in a counterdirection at the dryer felt nip. Such rejected air accumulates between the felt and the web and causes the web to separate by a significant amount from the felt. This separation can be observed to originate at the dryer felt nip and move progressively upstream towards the lead-in felt roll. The aforementioned separation can be prevented by applying a high level vacuum above the felt, just prior to the nip area, to evacuate the rejected air. Such high level of vacuum, which can compensate for the "nip rejection" phenomenon, cannot be readily applied along the full length of the felt as it would cause excessive wearing contact between the felt and the vacuum inducing boxes. Instead, the apparatus, according to the present invention, applies the high level vacuum only to the area immediately adjacent to the dryer felt nip.
Such vacuum is produced by means which is located in operative proximity to the area of the dryer felt converging nip, but on the opposite side of the felt from the web.
Therefore, it is a primary objective of the present invention to provide a transfer apparatus and method that overcomes the aforementioned inadequacies of the prior art proposals, and which makes a considerable contribution to the art of transferring a web from a press to a dryer section.